A proper install and maintenance is huge for longevity of furnaces, the duct work it connects to plays a large roll, undersized duct, not changing filters, lack of maintenance, etc all chip away at the lifespan of the furnace. A furnace can even be too big for your house. You get what you pay for with HVAC, good contractors will sell better equipment and stand behind it in the long run, the local contractor is the make or break for buying quality.